Method of producing fibers having porous structures

ABSTRACT

THERE ARE DISCLOSED IMPROVEMENTS IN THE PROCESS OF PREPARING POROUS FIBERS BY WET-SPINNING A SPINNING COMPOSITION CONTAINING DISPERSED GAS, THE IMPROVEMENTS INCLUDING PREPARATION OF SPINNING SOLUTIONS SATISFYING SPECIFIC MATHEMATIC RELATIONSHIPS AS TO SURFACE TENSION AND VISCOSITY W6THEREOF AND AS TO RELATIVE VOLUME OF SOLUTION AND GAS AS WELL AS LIMITING PARTICLE SIZE OF THE DISPERSED GAS BUBBLES.

United States Patent 3,836,616 METHOD OF PRODUCING FIBERS HAVING POROUS STRUCTURES Toshiyuki Kobashi, Tsukubo-gun, Okayama Prefecture,

and Noboru Abe, Okayama, Japan, assiguors to American Cyanamid Company, Stamford, Conn.

No Drawing. Filed July 9, 1973, Ser. No. 377,700 Claims priority, application Japan, Aug. 16, 1972, 47/ 81,977 Int. Cl. B2911 7/20 US. Cl. 264-50 Claims ABSTRACT OF THE DISCLOSURE There are disclosed improvements in the process of preparing porous fibers by wet-spinning a spinning composition containing dispersed gas, the improvements including preparation of spinning solutions satisfying specific mathematic relationships as to surface tension and viscosity thereof and as to relative volume of solution and gas as well as limiting particle size of the dispersed gas bubbles.

This invention relates to a process for producing fibers having a porous structure. More particularly, this invention relates to improvements in the method of producing porous fibers wherein a Spinning solution of a fiber-forming polymer into which has been dispersed an inert gas is wet-spun into fiber.

A number of methods for producing porous or hollow fibers have been previously proposed. In one previous method, a dispersion of inert gas in a spinning solution was spun into fiber. In another previous method, a volatile substance insoluble in both the polymer solvent and the coagulant is dispersed in the spinning solution which is then wet-spun into fiber and the dispersed volatile compound is then volatilized to provide the desired hollows for the porous structure.

However, when porous fibers are prepared by the first mentioned previous process, the inert gas was dispersed in the spinning solution by means of a colloid mill and it is extremely difficult to provide the dispersed bubbles with a suficiently fine diameter so as to enable spinning to be carried out without frequent filament breakage in spinning and fiber-stretching steps. Thus, it is not practical to maintain continuous spinning by this procedure.

In producing porous fiber by the second mentioned previous process, it is difficult to provide sufiicient hollows in the fiber formed to obtain the necessary porosity in the fibers and large amounts of volatile compound are required if the necessary hollows are to be achieved. The use of the large amounts of volatile compound not only adds to production costs but also imposes various processing difficulties. Use of large amounts of volatile liquid increases the difficulties of providing hollows of fine diameter and continuous processing is not practical. In addition the fiber produced is deficient in strength and elongation properties.

Accordingly, there continues to exist the need for processes or improvements therein for providing porous fibers whereby deficiencies of the previous processes are overcome.

In accordance with the present invention, there is provided in a process for preparing porous fibers by dispersing an inert gas in a spinning solution of a fiber-forming polymer, said spinning solution having a viscosity expressed in poises and measured at 50 C. and a surface tension expressed in dynes per centimeter and measured at 50 C and by wet-spinning the dispersion into fiber, the improvement which comprises employing as said spinning solution one which satisfies the following conditions; the

numerical value obtained by dividing the surface tension by the viscosity is in the range of about 0.5 to 10, the numerical value obtained by multiplying the surface tension by 0.25 power of the viscosity is in the range of 95 and 170, and the ratio of the volume of spinning solution to the volume of dispersed gas is in the range of about 5 to 25, said dispersed gas having an average particle size less than about 50 microns.

The process as improved by the present invention not only provides a continuously operative process for fibers of desirable porosity but also provides such porosity while maintaining high values of strength and elongation in the fiber provided.

The process of the present invention is applicable to any wet-spinning fiber-making process. More particularly the present process may be used to impart porous structure to fibers of viscose, polyvinyl alcohol, polyamides, polyvinyl chloride, and polyacrylonitrile and to fibers wherein these polymers are the main fiber-forming component when such fibers are produced by Wet-spinning. A preferred fiber type for use in carrying out the present invention is that designated as acrylic fiber and the invention will be more fully described in terms of acrylic fiber although it is to be understood that the invention is not limited thereto.

In carrying out the process according to the present invention, a fiber-forming acrylonitrile polymer containing at least weight percent, preferably weight percent, of acrylonitrile and any balance of at least one vinyl monomer copolymerizable therewith is employed. Such fiberforming polymers, or mixtures thereof, are well known in the art of wet-spinning acrylic fibers and do not need further identification herein since, per so, they form no part of the invention described.

The selected polymer is then dissolved in a conventional solvent therefor useful in wet-spinning. Such solvents may be selected from inorganic compounds such as concentrated aqueous solutions of salts represented by sodium thiocyanate, potassium thiocyanate, calcium thiocyanate, ammonium thiocyanate, zinc chloride, perchlorate salts of the metals named and of acids such as nitric and sulfuric. The solvents may also be selected from organic solvents represented by dimethyl formamide, dimethyl acetamide, ethylene carbonate, and dimethyl sulfoxide.

The concentration of polymer in the spinning solution using the solvents represented above will vary depending on the particular solvent employed, the molecular weight of the polymer selected and the viscosity of solution desired. Thus, the concentration cannot be precisely stated to cover each individual case but generally will be in the range of 5 to 25 weight percent.

In preparing the spinning solution, it is necessary to take into account the viscosity and surface tension of the final solution. These values are determined in accordance with standard procedures using 50 C. as the temperature of measurement. The viscosity value should be expressed in poises and the surface tension in dynes per centimeter. In order to be useful in the process of the present invention, the numerical value obtained by dividing the surface tension by the viscosity should be in the range of about 0.5 to 10.0. If the numerical value is higher than about 10.0, the bubbles of gas dispersed in the spinning solution are unstable and two or more tend to join together. Because of this instability of bubbles, it becomes necessary to reduce the holding time of the dispersion from preparation to spinning and this places impractical limitations on processing. Generally, it is impractical to provide a numerical value less than about 0.5 for surface tension divided by viscosity so that this value is the practical lower limit.

ice

The spinning solution must also have surface tension and viscosity values such that the numerical value obtained by multiplying surface tension by 0.25 power of the viscosity should be in the range of 95 and 170. If this numerical value is not in the range stated, it is necessary to employ excessive dispersing times or excessive dispersing forces to achieve the desired dispersion of fine bubble size. Such actions not only adversely affect economy of operation but also result in fiber of poor quality.

In addition to providing the numerical relationships of surface tension and viscosity of the spinning solution indicated above, it is also necessary to disperse sufiicient inert gas therein to provide a ratio of volume of spinning solution to volume of inert gas in the range of about 5 to 25. If the ratio is higher than about 25, there will be provided insufficient porosity for a desirable effect. If the ratio is lower than about 5, it becomes increasingly ditficult to prepare the desired dispersion and fiber properties tend to be adversely aifected.

In order to obtain the necessary mathematical relationships between surface tension and viscosity, proper selection of polymer and concentration thereof in the spinning solution can provide the necessary numerical values. Increase or decrease in polymer concentration can be employed to bring numerical values within the range of values specified. Use of a surface active agent can be used to reduce the surface tension of the spinning solution, when necessary, to bring the mathematical relationships into range. A combination of both variation of spinning solution concentration and use of a surface active agent may be used if desired. It should be noted, however, that use of a surface active agent is not necessary in the present invention and that, if used, the surface active agent must be used in conformity with the range of numerical values of the mathematical relationships of surface tension and viscosity of the spinning solution.

in preparing the dispersion, it is necessary to achieve dispersed gas bubbles that have an average diameter of less than about 50 microns, and preferably less than about 30 microns. Any intensive mixing equipment that. is capable of achieving such a gas dispersion in the spinning solution may be employed. A particularly suitable device is a socalled planetary gear mixer. Such device comprises a. central driving gear, in contact with which are two or more planetary gears which rotate about the drive gear and contacting the planetary gears in mesh therewith is the inner wall of the mixing chamber which confines the composition being mixed. Such a device, by the combined action of the various gears and confining chamber, produces much smaller particle sizes in shorter times at slower rotational speeds than can be obtained with other mixing devices. Using such device, the desired bubble diameter is readily provided.

As inert gases useful in the present process are those conventionally employed. Such gases include air, carbon dioxide, argon, neon, helium, and the like.

In preparing the dispersion for spinning, the necessary amount of inert gas in the proper state of subdivision is achieved. The dispersion is then spun into fiber by the conventional wet spinning procedures using a coagulant normally employed with the polymer solvent selected. The procedure will include the conventional steps of low temperature coagulation, stretching, drying, relaxing, and the like. The fiber as formed will contain fine bubbles of inert gas which will be retained in such form in the final fiber.

, The fiber obtained by the present invention possesses excellent fiber properties such as high strength and elongation and is porous due to pores formed uniformly along the axial direction of the fiber through the entire fiber structure. The pores to not vanish or become broken by being compressed in the various processing steps.

The fiber obtained by the present invention because of its porous structure finds a wide variety of uses in applications such as clothing and bedding in which uses their 4 light weight, resilience, protection against heat and cold are of great importance. Additional uses involve interior decorating and industrial applications.

When, in preferred embodiments of the process the planetary gear mixer described is employed, preferred conditions of operation are given by the relationship wherein D is the pitch diameter in meters of the driving gear, 11 is the rotational speed of the driving gear in revolutions per second, N is the number of planetary gears in a mixing layer, M is the number of gear layers provided, A is the thickness in meters of the planetary gears and Q is the flow rate of spinning solution in liters per second into which the inert gas is introduced.

In the planetary gear mixer, the central drive gear lies on a side with its meshed ends being horizontally disposed. A set of planetary gears lying in the same single plane and horizontally disposed engage the mesh of the drive gear forming a mixing layer. The planetary gears also engage the meshed inner wall of the chamber surrounding the mixer. A number of mixing layers may be provided by increasing the number of adjacent planes of planetary gears, the number of planes being determined by the size of the chamber and the thickness of the planetary gears added.

The present invention, as distinguished from conven tional processes of preparing porous fibers, provides stable dispersions of inert gas of fine bubble size and prevents the constant breaking of filaments during the spinning and stretching steps, thereby improving efficiency of the spinning operation. The present process enables increased porosity to be obtained in the fiber while at the same time providing improved strength and elongation compared to prior processes. Therefore, the industrial significance of the present process is extremely high.

In order to illustrate the present invention, the following examples are provided wherein all parts and percentages are by weight unless otherwise specifically indicated.

In the examples, the porosity of the fibers is indicated by the specific gravity of the fiber obtained. The specific gravity of conventional non-porous acrylic fiber is 1.18. In order for the fiber to possess satisfactory properties for use as porous fiber, it is necessary for its specific gravity to be less than 1.15.

The average diameter D of the gas bubbles on a surface area basis is calculated from the following formula:

wherein D represents the diameters of 200 bubbles in a sample of the spinning solution photographed microscopically.

EXAMPLE 1 The acrylonitrile polymer employed as the fiber-forming polymer consisted of a copolymcr of weight percent acrylonitrile and 10 weight percent methyl acrylate. A spinning solution was prepared at 11 weight percent of copolymer in concentrated aqueous sodium thiocy'anate.

The viscosity of the spinning solution at 50 C. was measured as 17 poises. The surface tension at 50 C. was measured to be 60 dynes per centimeter. The numerical value of surface tension divided by viscosity was 3.53. The numerical value obtained by multiplying the surface tension by the 0.25 power of the viscosity was 122.

After 5% by volume of air had been introduced into the spinning solution the spinning solution was processed in a planetary gear mixer to provide dispersions of air bubbles in the solution. The planetary gear mixer employed had five mixing layers in each of which the drive gear had a pitch diameter of 60 millimeters and eight planetary gears of pitch diameter of 20 millimeters,

6 EXAMPLE 3 The acrylonitrile polymer of Example 1 was again employed to prepare spinning solutions containing varying amounts of polymer. Air was introduced into the solu- 5 arranged between the driving gear and the inner wall of Hons at 5% by Volume- In 08 3 a d 5, 0.1% sorbrtan the chamber containing the solution, with a single driving monolaurate blased the Welght i solutlon' being common to each mixing layer. was added to ower e surface tension t ereof.

Eleven runs under varying conditions of operation of The mlxture P d as dlspefslon a the planetary gear mixer were made. The dispersions ob- Planetary gear mlxer operating with a drive gear rotatained in each run were wet spun into fib by conven. tronal speed of 300 revolutions per minute to treat the tional processing using a spinnerette having 100 orifices, mlXtllfe at a flOW rate of lltel' p minuteeach of 0.1 millimeter diameter, and an aqueous sodium The lsperslons were spun into fiber following the tbiocyanate coagulant. The gel filaments were stretched procedure of Example 1. The various details and specific at a stretch ratio of 2 at room temperature, were washed gravity of the fibers are given in Table II.

TABLE I Processing rate of spinning Dry fiber solution Q Rotational strength in mixer speed -l Run No (liters/sec.) D.,.a.N.A.M (r.p.m.) spinnability denier) 0. 005 1/36 300 No filament breakage.-. 3. 21 0.01 1/18 300 do a. 13 0.02 1/9 300 Occasional breakage-.- 2.93 0.02 1/18 600 No filament breakage 3. 02 0.03 1/12 600 o 3.12 0.04 1/9 600 Occasional breakage 3.08 0. 05 1/7. 2 600 Spinning impossible 0. 04 1/13. 5 900 N0 filament breakage... 3. 13 0.05 was 900 o 3.10 0.06 1/9 900 Occasional breakage... 3.02 0. 07 1/7. 7 900 Spinning impossible TABLE II Spinning composition Bubbles Polymer Surf. tens. Calculations Avg.bub- R0 cone. Viscosity ynes/ ble dia- Fiber, No. (percent) (poises) cm.) meter (It) Dispersed state spinnability Sp.G

9 5 55 11. 0 82 38 Bubbles joined to produce Frequent filament breakage;

gross bubbles. spinning becomes im ossible. 11. 6 17 60 3. 5 122 24 Fine and uniform Good- 1. 14 11.6 17 42 2.5 85 32 Sameasrunl Sameasrunl 13.4 55 66 1. 2 180 33 Uniform but large. Frequent filament breakage 1. 13.4 55 0.8 122 22 sameasrunz G00 1.14

1 Surf. tens., divided by viscosity. 9 Surf. tens., multiplied by viscosity with water, were stretched at a stretch ratio of 5 in boiling water, were dried in a current of air at 120 C. for 10 minutes, and were relaxed in saturated steam at 127 C. for 10 minutes. Strength values of the fibers obtained as well as dispersing conditions and spinnability are given in Table I.

From the data given in Table I, it can be seen that when the planetary gear mixer is operated under conditions that satisfy the relationship Q D -n-N-A-M spinnability of the dispersion is continuously achieved without filament breakage and high dry tensile strength in the fiber obtained is achieved.

EXAMPLE 2.

Using the spinning solution prepared in Example 1 above, 5% by volume of air was introduced into the spinning solution as fine bubbles in conjunction with an in-line homomixer. The rotational speed of the mixer had to be at 7000 r.p.m. to obtain an average bubble diameter of less than microns at a processing rate of 0.005 liters per second so as to provide a dispersion which could be spun into fibers without filament breakage during continuous spinning.

Although this example shows that alternative mixing devices can be employed in the process of the present process, it also shows the advantage of the planetary gear mixer in providing increased processing rate at lower rotational speeds.

What is claimed is:

1. In a process for preparing porous fibers by dispersing an inert gas selected from air, carbon dioxide, argon, neon, and helium in a spinning solution of a fiber-forming acrylonitrile polymer containing at least 70 weight percent acrylonitrile and any balance of at least one vinyl monomer copolymerizable therewith, said spinning solution having a viscosity expressed in poises and measured at 50 C. and a surface tension expressed in dynes per centimeter and measured at 50 C. and by wet-spinning the dispersion into fiber whereby the dispersed gas is retained as fine bubbles in the final fiber, the improvement which comprises employing as said spinning solution one which satisfies the following conditions: the numerical value obtained by dividing the surface tension by the viscosity is in the range of about 0.5 and 10, the numerical value obtained by multiplying the surface tension by the 0.25 power of the viscosity is in the range of and 170, and the ratio of the volume of spinning solution to the volume of dispersed gas is in the range of about 5 to 25, said dispersed gas having an average particle size less than about 50 microns.

2. The process of Claim 1 wherein the average particle size of the dispersed gas is less than about 30 microns.

3. The process of Claim 1 wherein the polymer solvent is a concentrated aqueous tbiocyanate solution.

4. The process of Claim 1 wherein the inert gas is air.

5. The process of Claim 4 wherein the ratio of the volume of spinning solution to the volume of air is 19.

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